A battery pack is an energy source consisting of one or more bank, for example, in series, with external connections (or taps) provided to power electrical loads, wherein each of the bank may further comprise one or more cells in parallel. Conventional battery packs typically employ symmetrical configurations, wherein the banks have the same nominal capacity measured in the unit of ampere-hour (Ah).
FIG. 1 shows an exemplary symmetrical battery pack 100, which has a “3S3P” configuration (wherein the nomenclature “3S3P” refers to a battery pack with 3 serially-coupled (S) banks per pack and 3 parallel (P) cells per bank). As shown, battery pack 100 includes three serially-coupled banks 105-1, 105-2 and 105-3. Each bank 105-1, 105-2 and 105-3 further comprises three cells 110 (e.g., 110-1, 110-2 and 110-3), 115 (e.g., 115-1, 115-2 and 115-3) and 120 (e.g., 120-1, 120-2 and 120-3) coupled in parallel. As they are formed by the same types of cells of the same quantity, banks 105-1, 105-2 and 105-3 provide the same capacity and/or voltage, and thus offer battery pack 100 a symmetrical configuration. Note that cells 110, 115 and 120 may have different capacities and/or voltages from each other. For example, cell 110 may have a relatively medium capacity (M), cell 115 may provide a relatively small capacity (S), and cell 120 may possess the largest capacity (L). Additionally, conventional, symmetrical battery packs typically provide only two taps. For example, battery pack 100 includes a positive tap PACKP, which is coupled to the positive electrode of battery pack 100's top bank 105-1, and a negative tap PACKN, which is coupled to the negative electrode of bottom bank 105-3. The negative tap PACKN may be electrically coupled to earth or be a floating point, which provides a voltage reference. In this disclosure, the negative tap PACKN is regarded as the ground node. Note that, with only two taps, battery pack 100 provides only one single voltage (with respect to the voltage reference, the ground node PACKN).
A conventional, symmetrical battery pack, such as battery pack 100, has several limitations. First, electrical loads of the battery pack may require different supply voltage levels. For example, a central processing unit (CPU) may require a supply voltage of 1V, while a universal serially-coupled bus (USB) port may need a supply voltage of 5V, 12V, 20V, or some other voltages. However, a conventional, symmetrical battery pack typically provides only one voltage. To address the problem, one may use DC-DC converters to regulate the battery pack's single voltage to appropriate voltages for individual loads. However, the deployment of DC-DC converters will inevitably result in losses. For a battery-powered system, such as a portable electronic device, those losses are critical. They shorten the device's operating life and may even cause over-temperature issues. In addition, DC-DC converter's efficiency is in an inverse relation to the ratio between the converter's input and output voltages. The higher the ratio between the converter's input and output voltages, the lower the efficiency that the DC-DC converter will be able to achieve. Therefore, the number of serially-coupled banks of a symmetrical battery pack has to be chosen carefully in to optimize the battery-powered system's overall efficiency. However, with only one voltage level available, the optimization presents a challenging task.
Second, battery-powered systems normally have significant size and space limitations based on the design constraints of their particular product implementations. It is desirable to have a flexible battery packaging so that it can make a full use of available space. For example, banks and/or cells of small sizes and/or irregular shapes can fill up space near a corner, edge, or curve of a device's outer shell, while banks and/or cells of large and/or regular size may only be able to be installed in a normal, e.g., central, position within the device. However, such banks and/or cells of varying sizes and/or shapes cannot fit in a conventional symmetrical battery pack.
Finally, even if a battery pack is designed with balanced banks, building banks with identical capacities and/or voltage is challenging because of variations in material and manufacturing processes. Even if symmetrical banks are manufactured, imbalance can arise over the life of the battery pack, as bank capacities and impedances may degrade with time and cycles. An imbalanced battery pack has reduced capacity because the bank with the highest state of charge will cause the charging process to terminate, which means that banks of a lower state of charge never get fully charged. Conversely, when the battery pack is discharged, the bank with the least charge can cause the discharging process to stop, even though charge may remain the other bank. Additionally, imbalanced banks may even present a safety risk, for example, from over-charging because of capacity imbalances. Note that the state of charge of a bank, as used herein, refers to the ratio between its remaining amount of charge and its rated capacity. The state of charge is measured in percentage points, where a 100% state of charge represents a fully charged bank and a 0% state of charge indicates a fully discharged bank.
One solution to address those limitations of symmetrical battery packs is to adopt an asymmetrical battery pack configuration, wherein banks within the battery pack may have different capacities and/or voltages and the banks may be accessed by electrical loads through multiple taps. In the asymmetrical configurations, special control algorithms may be employed to ensure that the asymmetrical banks reach the top of charge and bottom of charge during charging and discharging at the same time. Therefore, what is needed is a multi-tapped, asymmetrical battery pack designed to balance the states of charge among its asymmetrical banks during charging and discharging processes.